1. Field of the Invention
The present invention relates to an optical transmission network (hereunder called a CWDM network) which adopts a coarse wavelength division multiplexing (CWDM) system, and in particular to technology for realizing a connection between different CWDM networks using transmission paths or the like of an optical transmission network which employs a dense wavelength division multiplexing (DWDM) system (hereunder called a DWDM network).
2. Description of the Related Art
In the conventional DWDM network, as shown for example at the upper part of FIG. 9, double core one-way communication where a multi wavelength optical signal is combined in a single optical fiber and transmission is performed in the same direction, is common. Thus, when performing communication between terminals, two fibers are required. In the case of performing long distance transmission, a linear relay apparatus which includes an optical amplifier is provided between respective terminals, and relay transmission of DWDM light is performed while collectively amplifying optical signals which are propagated and attenuated on the optical fiber. The usage wavelength band width of the optical signal of the C-band and the L-band which is transmitted with a DWDM network, is approximately 30 nm for each band, and by adopting a network configuration where an erbium doped fiber amplifier (EDFA) is applied to the linear relay apparatus, the above long distance transmission of DWDM light becomes possible.
For the conventional CWDM network, as shown at the lower part of FIG. 9, single core two-way communication which performs mutual communication with a single optical fiber, is common. This is a low priced attractive network in which compared to the aforementioned DWDM network, the usage wavelengths (channels) are reduced, and moreover low cost transmitters or optical filters can be used since temperature control of the laser mounted on the transmitter is not performed (refer for example to Japanese Unexamined Patent Publication No. 2004-166300). In such a CWDM network, since the usage wavelength range is wide compared to the aforementioned DWDM network, the difference in the wavelength-dependent loss for each of the wavelengths (channels) after fiber transmission is great, and a difference occurs in the transmission distance due to the wavelength, so that the transmittable distance of the equipment constituting the network is restricted. More specifically, the equipment specification is matched with the wavelength of which the transmission distance is short. Furthermore, since temperature control of the laser is not performed, the usage wavelength width of the channel unit is wide (approximately 20 nm), which has the demerit in that an EDFA commonly used in DWDM networks cannot be used. Therefore, the CWDM network is not suitable in long distance transmission as with the DWDM network, and is principally applied to small scale networks.
Furthermore, as conventional technology related to add/drop nodes of a DWDM network, for example in Japanese Unexamined Patent Publication No. 2005-520434, in a DWDM network 10 as shown in FIG. 10, a technique is proposed where a remote node 120 for adding/dropping an optical signal of 1300 nm wavelength on the network is provided between any of a plurality of 1550 nm nodes 112, 114, 116, 118 for adding/dropping an optical signal of 1550 nm band on the network, and at the remote node 120, an optical signal of 1300 nm which is added on the network is sent to an adjacent 1550 nm node, and converted to an optical signal of 1550 nm band in a transponder, and transmitted on a DWDM network. According to this conventional technology, an operation is possible where the optical signal of 1300 nm added from the remote node 120 is converted to an optical signal of 1550 nm capable of long distance transmission on the DWDM network, and dropped at the desired 1550 nm node, or alternatively, the optical signal of 1550 nm band added from the 1550 nm node is converted to an optical signal of 1300 nm at the 1550 nm node adjacent to the remote node 120, and dropped at the remote node 120.
Incidentally, the conventional DWDM network and the CWDM network as described above normally each constitute an independent network, and since there are physical restrictions (for example specifications etc. for the usage wavelength band, the spectrum width of the light source, and the optical multiplexing/de-multiplexing filter) or differences in transmission characteristics, these do not become configurations which can communicate (carry) optical signals between different networks such as from CWDM to DWDM, or from DWDM to CWDM. However, there is a demand for connecting alternate CWDM networks and performing long distance transmission, in particular places or sections where CWDM networks are constructed, but corresponding with the long distance of CWDM networks has become a problem.
In the conventional technology related to add/drop networks of the aforementioned DWDM networks, an optical signal of 1300 nm applied to a remote node has been considered to replace the optical signal from the CWDM network to thereby enable carrying an optical signal between the DWDM network and the CWDM network. However, since one remote node only is arranged on the DWDM network, then for example it is difficult to use an optical transmission path of a DWDM network connected between a plurality of CWDM networks which are provided in different regions which are far apart. In other words, being able to directly add/drop CWDM light on a DWDM network applying the conventional technology is limited to single locations of remote nodes. Hence a plurality of CWDM networks cannot be connected on a DWDM network, and from the view point of correspondence with even longer distances for CWDM networks, this is insufficient.